What is the meaning of “this” in Java?

Question

Normally, I use this in constructors only.

I understand that it is used to identify the parameter variable (by using this.something), if i

Answer 1

It refers to the current instance of a particular object, so you could write something like

public Object getMe() {
    return this;
}

---

A common use-case of this is to prevent shadowing. Take the following example:

public class Person {
    private final String name;

    public Person(String name) {
        // how would we initialize the field using parameter?
        // we can't do: name = name;
    }
}

In the above example, we want to assign the field member using the parameter's value. Since they share the same name, we need a way to distinguish between the field and the parameter. this allows us to access members of this instance, including the field.

public class Person {
    private final String name;

    public Person(String name) {
        this.name = name;
    }
}

Answer 2

this refers to the current object.

Each non-static method runs in the context of an object. So if you have a class like this:

public class MyThisTest {
  private int a;

  public MyThisTest() {
    this(42); // calls the other constructor
  }

  public MyThisTest(int a) {
    this.a = a; // assigns the value of the parameter a to the field of the same name
  }

  public void frobnicate() {
    int a = 1;

    System.out.println(a); // refers to the local variable a
    System.out.println(this.a); // refers to the field a
    System.out.println(this); // refers to this entire object
  }

  public String toString() {
    return "MyThisTest a=" + a; // refers to the field a
  }
}

Then calling frobnicate() on new MyThisTest() will print

1
42
MyThisTest a=42

So effectively you use it for multiple things:

• clarify that you are talking about a field, when there's also something else with the same name as a field
• refer to the current object as a whole
• invoke other constructors of the current class in your constructor

Answer 3

I would like to share what I understood from this keyword. This keyword has 6 usages in java as follows:-

1. It can be used to refer to the current class variable. Let us understand with a code.*

Let's understand the problem if we don't use this keyword by the example given below:

class Employee{  
int id_no;  
String name;  
float salary;  
Student(int id_no,String name,float salary){  
id_no = id_no;  
name=name;  
salary = salary;  
}  
void display(){System.out.println(id_no +" "+name+" "+ salary);}  
}  
class TestThis1{  
public static void main(String args[]){  
Employee s1=new Employee(111,"ankit",5000f);  
Employee s2=new Employee(112,"sumit",6000f);  
s1.display();  
s2.display();  
}}  

Output:-

0 null 0.0
0 null 0.0

In the above example, parameters (formal arguments) and instance variables are same. So, we are using this keyword to distinguish local variable and instance variable.

class Employee{  
int id_no;  
String name;  
float salary;  
Student(int id_no,String name,float salary){  
this.id_no = id_no;  
this.name=name;  
this.salary = salary;  
}  
void display(){System.out.println(id_no +" "+name+" "+ salary);}  
}  
class TestThis1{  
public static void main(String args[]){  
Employee s1=new Employee(111,"ankit",5000f);  
Employee s2=new Employee(112,"sumit",6000f);  
s1.display();  
s2.display();  
}} 

output:

111 ankit 5000
112 sumit 6000

2. To invoke the current class method.

class A{  
void m(){System.out.println("hello Mandy");}  
void n(){  
System.out.println("hello Natasha");  
//m();//same as this.m()  
this.m();  
}  
}  
class TestThis4{  
public static void main(String args[]){  
A a=new A();  
a.n();  
}}  

Output:

hello Natasha
hello Mandy

3. to invoke the current class constructor. It is used to constructor chaining.

class A{  
A(){System.out.println("hello ABCD");}  
A(int x){  
this();  
System.out.println(x);  
}  
}  
class TestThis5{  
public static void main(String args[]){  
A a=new A(10);  
}}

Output:

hello ABCD
10

4. to pass as an argument in the method.

class S2{  
  void m(S2 obj){  
  System.out.println("The method is invoked");  
  }  
  void p(){  
  m(this);  
  }  
  public static void main(String args[]){  
  S2 s1 = new S2();  
  s1.p();  
  }  
}  

Output:

The method is invoked

5. to pass as an argument in the constructor call

class B{  
  A4 obj;  
  B(A4 obj){  
    this.obj=obj;  
  }  
  void display(){  
    System.out.println(obj.data);//using data member of A4 class  
  }  
}  

class A4{  
  int data=10;  
  A4(){  
   B b=new B(this);  
   b.display();  
  }  
  public static void main(String args[]){  
   A4 a=new A4();  
  }  
} 

Output:-

10

6. to return current class instance

class A{  
A getA(){  
return this;  
}  
void msg(){System.out.println("Hello");}  
}  
class Test1{  
public static void main(String args[]){  
new A().getA().msg();  
}  
}  

Output:-

Hello

Also, this keyword cannot be used without .(dot) as it's syntax is invalid.

Answer 4

As everyone said, this represents the current object / current instance. I understand it this way, if its just "this" - it returns class object, in below ex: Dog if it has this.something, something is a method in that class or a variable

class Dog {
private String breed;
private String name;

Dog(String breed, String name) {
    this.breed = breed;
    this.name = name;
}

public Dog getDog() {
    // return Dog type
    return this;
}

}

Answer 5

To be complete, this can also be used to refer to the outer object

class Outer {
    class Inner {
        void foo() {
            Outer o = Outer.this;
    }
  }
}

Answer 6

It refers to the instance on which the method is called

class A {

  public boolean is(Object o) {
    return o == this;
  }

}

A someA = new A();
A anotherA = new A();
someA.is(someA); // returns true
someA.is(anotherA); // returns false